/Engine/Source/Runtime/Engine/Classes/GameFramework/Character.h
Z velocity is zero during walking movement, and will be here as well. Another velocity can be computed here if desired and will be used when starting to fall.
void OnWalkingOffLedge ( const FVector & PreviousFloorImpactNormal, const FVector & PreviousFloorContactNormal, const FVector & PreviousLocation, float TimeDelta )